Influência do magnésio na resistência do arroz à mancha parda

Abstract

Brown spot, caused by the fungus Bipolaris oryzae, is one of the most important rice diseases. It is known that magnesium (Mg) has the potential to decrease the intensity of some diseases in crops of economic importance. Therefore, the aim of this study was to determine the effect of different Mg rates on rice resistance to brown spot. Rice plants (cv. Metica-1) were grown in nutrient solution containing 0.25, 0.5, 1, 2 and 4 mmol.L-1 Mg. The incubation period (IP), the number of lesions (NL) per cm2 of leaf, the brown spot severity, and the foliar concentration of Mg were evaluated. Data from severity were used to calculate the area under brown spot progress curve (AUBSPC). Another experiment with the rates of 0.25, 2.5 and 4 mmol.L-1 Mg was carried out to determined the concentration of malondialdehyde (MDA) and the activity of chitinase (CHI), β-1,3-glucanase (GLU), peroxidase (POX), polyphenoloxidase (PPO) and phenylalanine ammonia-lyase (PAL) in response to B. oryzae inoculation. There were linear positive and linear negative effects of the Mg rates on the foliar concentration of Mg and AUBSPC. There was quadratic effect of the Mg rates on both IP and NL per cm2 of leaf area. The highest concentration of MDA and high PPO activity were obtained with the rate of 4 mmol.L-1 Mg. The activities of CHI and GLU were not affected by the Mg rates. High POX activity occurred for plants supplied with 2.5 and 4 mmol.L-1 Mg. Significant increase in PAL activity occurred at 48 and 72 hours after inoculation at the rate of 4 mmol.L-1 Mg. The results of this study demonstrated that the high foliar concentration of Mg increased rice resistance to infection by B. oryzae mainly through the increase on the activities of defense enzymes.